1. Field of the Invention
The present invention relates to a semiconductor device and a method for manufacturing the same and, more particularly, to shallowing of a source/drain diffusion layer.
2. Description of the Related Art
Conventional MOSFETs include an n-channel MOSFET using electrons as carriers and a p-channel MOSFET using holes as carriers.
A source/drain region of an n-channel MOSFET is formed by doping an n-type impurity such as arsenic in a p-type silicon substrate, a p-type well region, a p-type epitaxial layer, or the like. A source/drain diffusion layer of a p-channel MOSFET is formed by doping a p-type impurity such as boron in an n-type well region, an n-type epitaxial layer, an n-type silicon substrate, or the like.
Since a CMOS semiconductor device in which the n-and p-channel MOSFETs are formed on the same chip (the same substrate) has various advantages such as low power consumption, it is frequently used.
The CMOS semiconductor device has been rapidly developed in recent years. In addition to n- and p-channel elements, the CMOS semiconductor device is expected to be further micropatterned. Since arsenic has small diffusivity, so-called "extension" of a diffusion layer caused by a thermal hysteresis during manufacturing of the diffusion layer is small. Therefore, the n-channel MOSFET is easily micropatterned.
However, in the p-channel MOSFET, since the diffusivity of boron is large, a source/drain diffusion layer largely extends in a direction perpendicular to the substrate surface and a direction parallel thereto. When the diffusion layer expands as a whole, the impurity concentration of the diffusion layer is decreased. When the concentration of the diffusion layer is decreased, the sheet resistance of the diffusion layer is increased. More specifically, when the impurity concentration of the diffusion surface is decreased, the contact resistance between the diffusion layer and an internal wiring layer is decreased.
In addition, when the diffusion layer expands as a whole, a channel length is decreased as a result, and a so-called "short-channel effect" is easily caused. In order to avoid this, in the p-channel MOSFET, the gate length of a gate electrode must be set to be larger than that of the n-channel MOSFET. This disables micropatterning of the p-channel MOSFET.
Diffusion of boron or arsenic constituting a source/drain diffusion layer is performed by annealing the source/drain diffusion layer in an oxygen atmosphere. However, when boron is diffused by annealing the diffusion layer in the oxygen atmosphere, a diffusion rate of the boron is increased (Oxidation Enhanced Diffusion: to be referred to as an OED effect hereinafter).